Preconditioning treatment to enhance redox tolerance of solid oxide fuel cells

ABSTRACT

A high temperature, redox tolerant fuel cell anode electrode and method of fabrication in which the anode electrode is pre-conditioned by application of an initial controlled redox cycle to the electrode whereby an initial re-oxidation of the anode electrode is carried out at temperatures less than or equal to about 650° C.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to high temperature fuel cells havingmetal-containing anode electrodes, in particular, solid oxide fuel cellsand, more particularly, to solid oxide fuel cell anode electrodes. Moreparticularly yet, this invention relates to solid oxide fuel cell anodeelectrodes that are redox tolerant, solid oxide fuel cells comprisingsuch electrodes, and a method for enhancing the redox tolerance of suchelectrodes.

2. Description of Related Art

Fuel cells are electrochemical devices that convert the chemical energyof a fuel into electrical energy with high efficiency. The basicphysical structure of a fuel cell consists of an electrolyte layer witha porous anode electrode and porous cathode electrode on opposed sidesof the electrolyte. In a typical fuel cell, gaseous fuels, typicallyhydrogen, are continuously fed to the anode electrode and an oxidant,typically oxygen from air, is continuously fed to the cathode electrode.The electrochemical reactions take place at the electrodes to produce anelectric current.

In a solid oxide fuel cell, the electrolyte is a solid, nonporous metaloxide, normally Y₂O₃-stabilized ZrO₂ (YSZ), the anode electrode is ametal/YSZ cermet and the cathode electrode is typically Sr-doped LaMnO₃.The solid oxide fuel cell operating temperature is typically in therange of about 650° C. to about 1000° C., at which temperature ionicconduction by oxygen ions occurs.

The most commonly used solid oxide fuel cell anode material is a poroustwo phase nickel and yttria stabilized zirconia (Ni/YSZ) cermet. Duringnormal fuel cell operation, this anode material remains a cermet.However, there are potentially several occurrences, such as air leakageinto the anode side of the fuel cell due to seal leakage, fuel supplyinterruption, and emergency stops, which may cause the anode electrodeto re-oxidize, forming an NiO/YSZ structure. Upon restarting of the fuelcell, the NiO/YSZ structure is chemically reduced to re-form the Ni/YSZanode electrode. However, this reduction and oxidation process (referredto as redox cycling) results in substantial bulk volume changes of theanode electrode. The bulk volume of a fully dense NiO sample would beexpected to contract by about 40.9% upon reduction and would be expectedto expand by about 69.2% upon oxidation. Although, due to expansion intothe pores, a NiO/YSZ solid oxide fuel cell anode electrode is unlikelyto experience such a drastic volume change, any volume change that doesoccur can have a significant effect on the integrity of other cellcomponents (e.g. electrolyte cracking) and cell component interfaces,which can, in turn, result in a significant degradation in theperformance of the fuel cell.

SUMMARY OF THE INVENTION

It is, thus, one object of this invention to provide a solid oxide fuelcell having enhanced tolerance to the effects of redox cycling.

It is one object of this invention to provide a solid oxide fuel cellanode electrode having enhanced tolerance to the effects of redoxcycling.

It is another object of this invention to provide a method for enhancingthe redox tolerance of solid oxide fuel cell anode electrodes.

It is another object of this invention to provide a method forfabrication of a redox tolerant solid oxide fuel cell anode electrode.

These and other objects of this invention are addressed by a solid oxidefuel cell anode electrode comprising a porous metal-YSZ structure havinga microstructure produced by applying an initial redox cycle to thestructure where the re-oxidation step of the cycle is carried out at atemperature less than or equal to about 650° C.

These and other objects of this invention are also addressed by a methodof fabricating a solid oxide fuel cell anode electrode comprising thesteps of forming a mixture of metal oxide particles and YSZ particlesinto a “green” or uncured anode structure typically with binders andplasticizers, heating the green anode structure in air to a suitablesintering temperature, forming a sintered anode structure comprising themetal oxide and YSZ, contacting the sintered anode structure with areducing agent at a temperature in the range of about 600° C. to about1000° C., forming a reduced anode structure having a firstmicrostructure, contacting the reduced anode structure with an oxidizingagent at a temperature in the range of about 400° C. to about 650° C.,forming an oxidized anode structure, and contacting the oxidized anodestructure with the reducing agent at a temperature in the range of about600° C. to about 1000° C., forming said reduced anode structure with asecond, redox tolerant, microstructure.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of this invention will be betterunderstood from the following detailed description taken in conjunctionwith the drawings wherein:

FIGS. 1 a, 1 b and 1 c show BSE (backscattered electron) SEM (scanningelectron microscope) images of a sintered (a), reduced (b) andre-oxidized (c) solid oxide fuel cell anode electrode;

FIGS. 2 a, 2 b, 2 c and 2 d show TEM (transmission electron microscope)images of a sintered (a), reduced (b), re-oxidized (c) and rereduced (d)solid oxide fuel cell anode electrode;

FIG. 3 is a diagram showing the results of a thermomechanical analysis(TMA) of solid oxide fuel cell anode electrode samples during oxidationat 600° C. and 750° C.;

FIG. 4 is a diagram showing voltage degradation after redox cycling fora baseline solid oxide fuel cell;

FIG. 5 is a diagram showing the cumulative degradation of a solid oxidefuel cell after redox cycling at various temperatures;

FIG. 6 is a diagram showing a comparison of the redox tolerance of asolid oxide fuel cell having a preconditioned anode electrode inaccordance with one embodiment of this invention and a baseline solidoxide fuel cell; and

FIG. 7 is a diagram showing a cumulative % degradation comparison ofsolid oxide fuel cells having a baseline anode electrode and solid oxidefuel cells having preconditioned anode electrodes in accordance withthis invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Although the invention is described herein in the context of solid oxidefuel cells and anode electrodes therefor, it will be appreciated bythose skilled in the art that the basic principles of this invention maybe advantageously applied to other high temperature fuel cells employingmetal-containing anode electrodes that may be subject to redox cycling,and such other fuel cells and fuel cell components are deemed to bewithin the scope of this invention.

The invention disclosed herein is an anode electrode for a solid oxidefuel cell and a method for fabrication and preconditioning of the anodeelectrode which addresses the problems associated with redox cycling ofthe solid oxide fuel cell. FIGS. 1 a, 1 b and 1 c show BSE SEM images ofsolid oxide fuel cell anodes in an as prepared, or sintered, state,after undergoing reduction by contact with a reducing gas, and afterre-oxidation, respectively. FIGS. 2 a, 2 b, 2 c and 2 d show TEM imagesof solid oxide fuel cell anodes in an as prepared state, afterundergoing a first reduction, after re-oxidation, and after a secondreduction, respectively. As can be seen from these figures, redoxcycling of a solid oxide fuel cell anode electrode irreversibly altersthe anode electrode microstructure after the first redox cycle. As usedherein, the first, or initial, redox cycle comprises the initialreduction of the sintered metal oxide/YSZ anode structure to produce thereduced anode structure followed by the initial re-oxidation of thereduced anode structure.

Electron microscopy of the anode electrode shows that, in the asprepared, i.e. sintered, condition, the anode electrode comprises NiOparticles several microns in size, YSZ grains about one micron in sizeand intergranular porosity. After the first reduction, the overall Nigrain size remains about the same as the consumed NiO and epitaxialgrowth of Ni crystals on NiO grains is observed. The amount ofintergranular porosity increases and very fine, 50 nanometer (nm),intergranular pores are formed throughout the Ni grains. This increasein the amount of porosity is due to the large volume change that occurswhen NiO is reduced to Ni. When the anode electrode samples arere-oxidized, the NiO particles in the SEM images appear spongy with muchsmaller intergranular pores than the as prepared anode electrodesamples. The re-oxidized anode electrode comprises smaller (less thanabout 100 nm), randomly oriented grains of NiO. The grain refinementthat occurs upon re-oxidation is likely due to the large number ofintragranular pores that occur upon reduction which serve as nucleationsites. Anode electrode samples reduced for a second time were also veryfine grained (less than about 200 nm) and contained significant amountsof small intergranular porosity. The YSZ grains were unaffected by theredox cycles. The grain refinement and microstructural changes thatoccur after the first re-oxidation cycle significantly alter the anodemicrostructure. These changes also occur after every subsequent redoxcycle, but the resulting microstructure is similar to the microstructureresulting from the initial re-oxidation. Thus, a new “redox cycled”equilibrium microstructure is formed after the first redox cycle.

Notwithstanding, we have found that subsequent redox cycling of solidoxide fuel cells in which the anode electrode has been preconditioned inaccordance with this invention does not significantly affect theintegrity of the other fuel cell components and fuel cell componentinterfaces and, thus, does not result in a significant degradation inthe performance of the fuel cell.

FIG. 3 shows the results of a thermomechanical analysis (TMA) of solidoxide fuel cell anode electrode samples subjected to oxidation at 600°C. and 750° C. As can be seen, the rate and amount of oxidation-inducedexpansion of the anode electrode sample was substantially reduced at600° C. compared to expansion of the anode electrode at 750° C. Byreducing the oxidation-induced expansion of the anode electrode, theundesirable impact of the expansion on the integrity of other fuel cellcomponents and of the fuel cell component interfaces and, thus, on thecell performance degradation is significantly reduced.

The amount of electrochemical performance degradation of a solid oxidefuel cell after redox cycling was characterized using a single celltesting facility. The initial performance of the fuel cell wascharacterized, after which air was blown over the anode electrode forvarious amounts of time in order to re-oxidize the anode electrode. Theanode electrode was then reduced and the electrochemical performance ofthe fuel cell was measured again. The results are shown in FIG. 4.Baseline testing of fuel cell redox tolerance showed that significantelectrochemical performance degradation occurs at redox times greaterthan about 60 minutes, corresponding to a redox depth of about 30%, andthat the greatest amount of redox-inducing degradation occurs after thefirst redox cycle to a 100% redox depth, occurring after 3.5 or morehours. 100% redox depth corresponds to all of the nickel in the reducedanode electrode being re-oxidized.

FIG. 5 shows the results of single cell tests with redox cyclesperformed at temperatures less than about 750° C. As can be seen,lowering the anode electrode oxidation temperature significantly loweredthe amount of electrochemical performance degradation after redoxcycling.

Thus, the combination of the TMA, SEM/TEM, baseline redox single celltests and lower temperature single cell tests clearly suggests that theredox tolerance of the cell may be enhanced by a low temperatureoxidation treatment, which acts to condition the microstructure of theanode electrode.

FIG. 6 shows a comparison of cumulative percent electrochemicalperformance degradation versus redox time for a baseline redox cell testand a preconditioned cell test, that is, a test in which pre-oxidationof the electrode is carried out at temperatures less than about 600° C.As can be seen from the figure, preconditioning the anode electrodemicrostructure significantly enhances the cell redox tolerance comparedto baseline test cells. The results represent an average of three testsfor the baseline test cells and two tests for the pre-oxidized testcells. All tests were carried out at 750° C. and 0.74 A/cm².

Whereas FIG. 6 shows cumulative percentage degradation, Table 1 showsthe data in numerical form as a percentage of voltage degradation perredox cycle. Comparison of the data easily shows that the first fullredox cycle (greater than 100% oxidation depth, which means that moreair is supplied to the electrode than is needed to oxidize all of thenickel in the electrode) causes the most degradation for baseline cellsat −4.1% (Table 1). This is still the case for the pre-oxidized cell,but the value is only −0.9% degradation, thereby clearly showing thatthe pre-oxidation of the electrode conditions the electrodemicrostructure to lower further degradation. TABLE 1 Comparison ofDegradation from Thermal Cycling for Baseline and Pre-oxidized CellsRedox Time Redox Depth Degradation (%) Degradation (%) (min) (%)Baseline Cells Pre-oxidized Cells Initial 0 0 0.0 0.0 Redox 1 20 10 −0.1−0.3 Redox 2 40 20 −0.2 −0.2 Redox 3 60 30 −0.7 −0.2 Redox 4 120 60 −1.6−0.3 Redox 5 240 120 −4.1 −0.9 Redox 6 360 180 −1.7 −0.7

FIG. 7 shows the results of individual cell tests at the same conditionfor a range of pre-oxidized cell tests and a baseline test comparison.Several of these tests incorporate other redox enhancements, but itstill can be seen that pre-oxidation alone produces the lowest redoxdegradation. This is likely due to interference with the pre-oxidationprocess from the other redox enhancements.

A key element for fabrication of a solid oxide fuel cell anode electrodein accordance with one embodiment of the method of this invention issubjecting the anode electrode metal oxide to a controlled initial redoxcycle where the initial re-oxidation step takes place at temperatures ofless than about 650° C. There are several stages in the fabrication ofthe anode electrode and/or fuel cell at which the controlled cycle maybe applied. In accordance with one particularly preferred embodiment ofthis invention, the initial controlled redox cycling is carried outin-situ, that is, with the anode electrode as a component of anassembled fuel cell. Alternatively, in accordance with one embodiment ofthis invention, the initial redox cycling is carried out on the anodeelectrode structure prior to assembly of the fuel cell. In accordancewith yet another embodiment of this invention, the redox cycling isapplied to the mixture of metal oxide/YSZ particles prior to formationof the green anode structure or to the metal oxide powders alone priorto mixing with YSZ.

While in the foregoing specification this invention has been describedin relation to certain preferred embodiments thereof, and many detailshave been set forth for the purpose of illustration, it will be apparentto those skilled in the art that the invention is susceptible toadditional embodiments and that certain of the details described hereincan be varied considerably without departing from the basic principlesof this invention.

1. A method for preconditioning a solid oxide fuel cell anode electrodecomprising the steps of: subjecting a sintered said anode electrode toan initial redox cycle in which re-oxidation is carried out at atemperature less than or equal to about 650° C.
 2. A method inaccordance with claim 1, wherein said sintered anode electrode comprisesa metal oxide and zirconia.
 3. A method in accordance with claim 2,wherein said zirconia is stabilized with yttria.
 4. A method inaccordance with claim 2, wherein said metal oxide is NiO.
 5. A method inaccordance with claim 1, wherein said redox cycle is carried out to atleast a 100% redox depth.
 6. A method in accordance with claim 1,wherein said temperature is in a range of about 400° C. to about 650° C.7. A method of fabricating a solid oxide fuel cell anode electrodecomprising the steps of: forming a mixture of metal oxide particles andYSZ particles into a green anode structure; sintering said green anodestructure, forming a sintered anode structure; contacting said sinteredanode structure with a reducing agent at a reducing temperature in arange of about 600° C. to about 1000° C., forming a reduced anodestructure having a first microstructure; contacting said reduced anodestructure with an oxidizing agent at an oxidizing temperature in a rangeof about400° C. to about 650° C., forming an oxidized anode structure;and contacting said oxidized anode structure with said reducing agent atsaid reducing temperature, forming said reduced anode structure with asecond microstructure.
 8. A method in accordance with claim 7, whereinsaid metal oxide is NiO.
 9. A method in accordance with claim 7, whereinsaid second microstructure is redox tolerant.
 10. In a solid oxide fuelcell having a metal-cermet anode electrode, a method for enhancing redoxtolerance of said solid oxide fuel cell comprising the steps of:contacting said metal-cermet anode electrode with an oxidizing agent ata temperature in a range of about 400° C. to about 650° C., forming anoxidized anode electrode; and contacting said oxidized anode electrodewith a reducing agent at a reducing temperature in a range of about 600°C. to about 1000° C., forming a preconditioned metal-cermet anodeelectrode.
 11. A method in accordance with claim 10, wherein saidoxidizing agent is provided in an amount sufficient to provide at leasta 100% redox depth.
 12. A method in accordance with claim 10, whereinsaid metal-cermet comprises Ni and YSZ.
 13. In a solid oxide fuel cellhaving an anode electrode, a cathode electrode and a solid electrolytedisposed between said anode electrode and said cathode electrode, theimprovement comprising: said anode electrode preconditioned by aninitial re-oxidation at a temperature less than or equal to about 650°C.
 14. A solid oxide fuel cell in accordance with claim 13, wherein saidanode electrode comprises Ni and YSZ.
 15. A solid oxide fuel cell inaccordance with claim 13, wherein said anode electrode is preconditionedto at least a 100% redox depth.
 16. A solid oxide fuel cell inaccordance with claim 13, wherein said temperature of said initialre-oxidation is in a range of about 400° C. to about 650° C.
 17. A solidoxide fuel cell in accordance with claim 13, wherein said anodeelectrode is preconditioned in-situ.
 18. A solid oxide fuel cell inaccordance with claim 13, wherein said anode electrode is preconditionedprior to assembly into said solid oxide fuel cell.
 19. A solid oxidefuel cell anode electrode comprising: a porous metal-YSZ structurehaving a microstructure produced by initially re-oxidizing saidstructure at a temperature less than or equal to about 650° C.
 20. Asolid oxide fuel cell anode electrode in accordance with claim 19,wherein said metal is Ni.
 21. A solid oxide fuel cell anode electrode inaccordance with claim 19, wherein said temperature of said initialre-oxidizing is in a range of about 400° C. to about 650° C.